It is known in the art to mount a transformer core along with the primary and secondary coils in a solid cast material to obtain a "dry" transformer, i.e. without using a dielectric liquid or gas to dissipate the heat generated in the coils and in the transformer core. Success in making such solid or dry transformers has only been found to a limited extent in relatively low power transformers.
Some of the difficulties encountered will be briefly discussed in the following. Heat dissipation through a solid dielectric material is known to be poor and the result of thermal build-up can create hot spots or high thermal gradients which can crack the solid dielectric material. The resulting fissure or fissures can be dangerous because of mechanical instability (the transformer body can break apart), and a break-down in the dielectric medium between the coils, and the core or ground.
Furthermore, if arcing occurs within the solid insulation, vaporization of the solid material can build up gas pressure which can even lead to fragmentation of the solid material and explosion.
Conventional dry-type transformers require a grounded enclosure to remove any electrical shock hazard. Such enclosures are typically metal cages having dimensions much larger than the transformer itself, making installation space requirements difficult.
Another problem in constructing a solid or dry transformer is that the expansion and contraction of the core as a result of temperature variations and shrinking of the cast solid insulation material induces stresses on the cast transformer body.
A further difficulty in manufacturing a large scale transformer, such as a distribution transformer, having a surrounding dielectric cast material lies in curing or setting the cast material in an even and homogeneous way to provide for homogeneous physical properties throughout the solid cast body.